1. Field of the Invention
The present invention relates to friction clutches with a low declutching force, especially for motor vehicles.
2. Description of Related Art
As is known, in a conventional clutch interposed between a driving shaft and a driven shaft, a diaphragm bears on the base of a cover plate, which is fixed to a rotary driving flywheel, so as to displace a pressure plate towards the said flywheel which constitutes a reaction plate, in order to grip the friction liners of a clutch friction wheel between the said pressure and reaction plates.
The driving flywheel is fixed in rotation to a first shaft such as a driving shaft, while the clutch friction wheel has at its inner periphery a hub for mounting it on a second shaft, such as a driven shaft, for rotation therewith.
The clutch is accordingly normally engaged, or clutched, with torque transmission between the driving and driven shafts.
In order to disengage, or declutch, the clutch, it is necessary to act axially with the aid of a clutch release bearing, by pushing in the case of a clutch of the push-to-release type, on the inner end of the fingers of the diaphragm in order to cause the said diaphragm to pivot and overcome the force which is exerted by the diaphragm on the pressure plate, which is movable axially so as to release the friction liners. The torque is then no longer transmitted from the driving shaft to the driven shaft, because the friction liners are no longer gripped between the pressure and reaction plates which are fixed to the cover plate for rotation with the latter.
Typically, the cover plate, the pressure plate and the diaphragm constitute a unitary assembly referred to as a clutch mechanism, which is assembled through its cover plate on the flywheel, with resilient tongues coupling the pressure plate to the cover plate for rotation together but with axial mobility.
As is known, the diaphragm has a central hole and includes an external peripheral portion in the form of a Belleville ring which is extended inwardly by a central portion divided into radial fingers by slots.
The diaphragm bears on the cover plate and on the pressure plate through its Belleville ring portion. Thus, the declutching device of the clutch, which enables the action of the resilient clutch engaging means to be counteracted at will, consists of the fingers of the diaphragm, on the inner ends of which the clutch release bearing acts, while the Belleville ring of the diaphragm constitutes axially acting resilient clutch engaging means for gripping the friction liners between the pressure and reaction plates, and therefore for biasing the pressure plate axially away from the base of the cover plate.
In the free state, the diaphragm is frusto-conical in form. Once it is mounted in the clutch, its Belleville ring is mounted under prestress and is more or less flat. During the declutching operation or operation of disengaging the clutch, the conicity of its Belleville ring is changed.
As is known, the characteristic curve of the diaphragm which represents the force exerted as a function of the course of declutching travel, taken for example at the level of the inner ends of the fingers of the diaphragm, is determined by the dimensions of its Belleville ring, and especially by the ratio between the height of the frustum of a cone of the Belleville ring in the free state and the thickness of the diaphragm.
This characteristic curve generally passes through a maximum.
Thus, the force to be exerted on the inner ends of the fingers of the diaphragm during the declutching operation increases to a maximum value, diminishes gradually down to a minimum, and then increases once again.
The difference between the maximum and minimum values may be large. For more detail as to this characteristic curve, reference should be made for example to the document FR-A-1 392 569.
In the document FR-A-1 392 569, a resilient progressive engagement device is provided outside the clutch friction wheel, for preventing the above mentioned maximum value being passed through during the declutching operation. This device is arranged in series with the Belleville ring of the diaphragm, and has an elastic force which is significantly less than that of the Belleville ring. The device has a course which is limited between a prestress position in which its force is at a maximum, and a stress position in which its force is at a minimum.
With this arrangement, a generally increasing declutching force is obtained at the clutch release bearing.
It is not possible to obtain, during the declutching operation, as great an assistance as is desired, since the progressive engagement device reduces the force exerted by the diaphragm on the pressure plate during the declutching operation.
The load curves of the progressive engagement device, which is typically mounted within the clutch friction wheel, and of the diaphragm, have profiles which do not enable them to be combined so as to obtain a high degree of assistance in declutching, in particular if it is desired to have regard to the compatibility of courses of declutching travel, and declutching forces, with current declutching control means. In addition, the profile of the curve of the progressive engagement device changes very rapidly as the clutch becomes older, and this cancels out the assistance given to the declutching operation to an extent that is greater, the higher the demand for such assistance. In order to prevent the curvature of the assistance device becoming set with ageing of the latter, it is preferable to locate it in a position in which its stiffness curve will remain stable over time, and in particular, in a position in which it is placed outside the zone of the progressive engagement of the friction wheel, in which liner contact and incrustation effects develop and cause the progressive engagement curve to change.
Consideration may then be given to providing an assistance spring, to act in parallel with the diaphragm. In that case, the assistance spring, which may for example be in the form of a Belleville ring, can engage on the cover plate and on the inner ends of the fingers of the diaphragm.
In the engaged condition of the clutch, it may then exert a minimum force, and then, as its conicity varies, exert an assistance force during the declutching operation; this assistance force is preferably at a minimum value in the engaged condition of the clutch.